A conventional mobile communications system employs a line switching type communications method, where a dedicated channel is allocated to a user. Such a method is suitable for a system which focuses on an interactive service of voice, moving image, or the like (See Non-patent Publication 1, for example) However, since traffic will be transmitted in a bursty manner as IP packets due to implementation of the Internet Protocol (IP) in a core network of future mobile communications systems, packet transmission is desirable also in radio transmission. In addition, when packet transmission is implemented in the radio transmission, delay in the radio transmission needs to be reduced, required transmission power needs to be reduced, and link capacity needs to be increased, for example. Moreover, error reduction in the radio transmission and highly reliable packet transmission are taken into consideration.    Non-patent Document 1: “Advanced Digital Mobile Communications”, edited by Keiji Tachikawa, Kagaku-shimbun-sha., published in January 2002, pp. 160-178.